1. Field of the Invention
This invention relates to a microwave freqency discriminator and more particularly to a discriminator including a field effect transistor.
2. Description of the Prior Art
Techniques and devices for the rapid and accurate determination of an unknown signal frequency are of significant interest in modern communication systems, in particular, for electronic counter measure (ECM) systems operating at microwave frequencies. Microwave freqency discriminators capable of converting incoming unknown frequencies into voltages for processing are often used in ECM systems. A microwave frequency discriminator has been defined as a circuit that provides an output voltage which is related to and usually proportional to the frequency of the incoming signal. The discriminator voltage output versus frequency response, commonly termed the "discriminator characteristic," is the response in which the output voltage varies nearly linearly with respect to frequency over a predetermined frequency bandwidth. The bandwidth is generally determined by the slope, linearity, and resolution of the discriminator and is the frequency range over which it provides an unambiguous voltage output which is related to input frequency.
A typical prior art broadband microwave discriminator, such as shown by FIG. 1, utilizes passive components such as transmission lines or an arrangement of lumped elements to vary the level of the output power as a function of frequency. Such a discriminator terminated with a square law detector will provide D.C. voltage output proportional to the frequency of the incoming RF signal. A square law detector is defined as a device having a transfer characteristic V.sub.out = k V.sub.in.sup.2 where k is proportionality factor, such that the output voltage, V.sub.out is proportional to the power of the incoming signal, V.sub.in.sup.2.
Most discriminators are preceded with a limiter to provide a constant power input to the discriminator, whereby the output voltage from the discriminator is a function of frequency alone. The limiter can be eliminated, however, if the incoming signal has a constant magnitude. Since such a prior art passive discriminator network has no gain, the voltage output which it can produce is restricted by the power available from the limiter preceding the discriminator. The overall linearity and usable bandwidth of the discriminator is also restricted by the frequency response and uniformity characteristics of each of the many elements of such a passive discriminator. In addition, the numerous interconnections in the passive discriminator, as shown in FIG. 1, produce multiple wave reflections which cause distortions of the frequency-voltage discriminator characteristic which reduce the accuracy and usuable frequency range of the discriminator.
As depicted in FIG. 1 of the drawing, a passive-element prior art discriminator 10 comprises a limiter 12 which receives an incoming RF signal 14 of unknown freqeuency and variable power level and converts signal 14 to an RF signal 16 of constant power level. Signal 16 is applied to a 3 db hybrid coupler 18 which splits signal 16 into two signal components. One component of the split signal is transmitted through a short path 20 and the other component through a longer path 22. The different path lengths produce different phase shifts of the two signal components. The split signals are recombined in another 3 db hybrid coupler 24. The power levels of the two outputs of the 3 db hybrid coupler 24 vary with frequency as a result of the vector summation of the two split signals having a differential phase shift. Detector diodes 26 and 28 are utilized to demodulate the frequency dependent signal received from coupler 24 and convert the signal into a D.C. voltage. A video amplifier 30 may be used to sum or compare and amplify the D.C. signal for subsequent measurement or display.